Method or communications system using a robust diversity combination

ABSTRACT

A method transmits packet data in a communications system between a transmitter and a receiver. One potential goal is to ensure correct processing of such packet data. To this end, the packet data are repeatedly transmitted by the transmitter and are received by the receiver as data received earlier or data received later. A diversity combination of the data received earlier and the data received later is performed to reconstruct the transmitted data packet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/DE01/02434 filed on 29 Jun. 2001 and GermanApplication No. 100 31 677.8 filed on 29 Jun. 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for performing a diversity combinationin a communications system.

In radio communications systems, messages and information, for examplevoice, image information or other data, are transmitted usingelectromagnetic waves between the transmitting and receiving station(base station and subscriber station respectively) via a radiointerface. In existing mobile radio communication networks conforming tothe GSM standard (GSM: Global System for Mobile Communication), new dataservices such as a packet data service GPRS (General Packet RadioService) are also currently being introduced. In addition to thetransmission of voice data, third-generation communications systems,e.g. the UMTS (Universal Mobile Telecommunication System) in accordancewith the UTRA standard (UTRA: Universal Terrestrial Radio Access), alsoprovide for the transmission of packet data units (PDUs). The packetdata units are derived by segmenting and by adding additional controlinformation from large data packets of higher layers or system levels(e.g. Layer 3). In particular, packet data is transmittedasynchronously, or not synchronously, with the result that thetransmission durations and/or the transmission paths of individualpacket data units transmitted in succession may differ from one other.To identify the packet data units arriving at the receiver, the unitsare provided with packet data identification information or sequencenumber. In the systems currently used or proposed, the identificationinformation is transmitted in the header section (header) of the datapacket or as transport format combination identifiers (TFCI).

The correct transmission of the identification information is veryimportant here since the data packets are later frequently combined intolarge blocks, and incorrect identification information would lead to anincorrect combination of individual data packets. In the worst case,such an incorrect combination is not detected until very late, or evennot at all, so that a renewed request to transmit the packet data isissued very late, or even incorrect data are further processed. Inparticular, a non-detected processing of incorrect identificationinformation leads to serious disruption of the traffic at air interfacesbetween the transmitter and receiver, for the most part withlong-lasting consequences.

Since data losses can occur in a multiplicity of situations during thetransmission of the packet data units, data protection methods fortransmitted data are known. In particular data protection includesencoding methods and repetition methods, e.g. an automatic datarepetition method with combinable encoding for forward error correction(FEC) known for short as Hybrid ARQ Type I or II (ARQ: Automatic RepeatRequest), which are sketched in FIG. 1. Following a first unsuccessfuldata transmission of a packet data unit PDU, a renewed transmission isrequested (ACK/NACK) by the receiving station MS from the transmittingstation BS. While the first transmission can be performed with optionalencoding (P1), encoded data P2 are transmitted at least for therepetition. In this case the redundancy can increase from repetition torepetition, with a corresponding increase in the probability that thedata packets PDU can be correctly reconstructed at the receiving end.

FIG. 1 shows three ways of obtaining corrected data. The errorcorrections that are based directly on only the received polynomials P1and P2 correspond to the ARQ-I method, in which different polynomials P1and P2 need not necessarily be selected. With the ARQ-I method, thereceiver does not need to store the data from earlier transmissions inorder to decode the current transmission. The error correction shown inthe middle of FIG. 1 combines or chains in accordance with the ARQ-IImethod the data of the two received polynomials P1 and P2. If the twopolynomials P1 and P2 are identical, the encoded data can be combined,which is referred to as diversity combining. If the two polynomials P1and P2 are different on the other hand, the receiver cannot combine theassociated transmissions, but should chain them to form a combined“larger” encoding block, which is subsequently decoded. With diversitycombining, in the case of the incorrect reception of a plurality ofdifferently encoded data polynomials, the transmission of the datapolynomial Pi that was received with the lowest signal-to-noise ratio ispreferably requested again.

In connection with the hybrid ARQ error correction, a diversitycombination can be used, e.g. a maximum ratio combining (MRC), if copiesof earlier transmitted encoded data of the same polynomial are repeated.With increasing redundancy transmission, that is to say with theincreasing number of repeated transmissions with stronger encoding ineach case, however, the following errors can occur in the receivingstation:

1. If the signaling information is transmitted in-band in the headersection of the transmitted data block, that is to say together with thepayload data, and is not protected here with an individual checksum(CRC: Cyclic Redundancy Check) to save system capacities and hencecosts, then undetected errors can occur when decoding the headersection. If the identification information of the data packet or of theheader section was corrupted or lost, then the corresponding data block(with diversity combining or chaining) is processed with data from otherdata packets or data blocks not belonging to it. In particular, this canlead to the destruction of valuable buffered baseband information in thereceiving station. In the worst case, such errors can even propagateunnoticed, or only noticed at a late stage.

2. If the receiving station attempts to decode a data block transmittedby an alien transmitting station and received as a result of anoverreach, comparable problems may arise.

In general it can be observed that as a rule the above methods offergood results if the noise signals superimposed on a data block can bedescribed as Gaussian distributed noise. In mobile radio communications,however, signaling of alien transmitting stations or data transmittedover other paths can lead to interference that does not correspond toany Gaussian distribution. Such interference cannot be readily handledusing methods such as the maximum ratio combination MRC. In particularoutliers that are superimposed on the wanted receive signal with highamplitudes also make a negative impact as noise signals. One specificproblem of the maximum ratio combination MRC is the necessity ofestimating the signal-to-noise ratios from the transmitted data prior tothe actual combining. It is no trivial matter to obtain a stableestimate of signal-to-noise ratios when there are outliers present inthe transmitted data.

A code combining in which a code rate is adapted to prevailing channelconditions is known from the publication “Cide Combining—A MaximumLikelihood Decoding Approach for Combining an Arbitrary of NoisyPackets”, David Chase, IEEE Transactions on Communications, IEEE INC.New York, US, VOL.COM-33, No. 5, May 1985, pages 385–393.

A data transmission in which a data packet with errors is transmittedagain is known from WO 99/26371.

SUMMARY OF THE INVENTION

One possible object of the invention is to provide a method for thetransmission of packet data units or data packets in a radiocommunications system, and to provide such a radio communications systemwhich permits a more stable diversity combination.

The application of the robust maximum ratio combination offers both verygood performance in the case of Gaussian noise signals and a more stableperformance given a variety of, in particular non-Gaussian, noisesignals and other error situations.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 shows schematic flowcharts for data transmission and dataprocessing with hybrid ARQ methods, and

FIG. 2 schematically shows the structure of a known radio communicationssystem by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The communications system illustrated in FIG. 2 shows a radiocommunication network having devices that enable a packet data serviceGPRS. An example of a stationary or mobile communications terminal is amobile station MS of a mobile subscriber that is wirelessly connectedvia an air interface V to devices of a terrestrial UMTS radio networkUTRAN (UMTS Terrestrial Radio Access Network), or respectively to itsbase station system BSS with fixed base stations BS and base stationcontrollers. The connection to a packet-oriented communication networkGPRS-N is realized by the UMTS radio network UTRAN via a mobileswitching center MSC. The communication network GPRS-N has devices thatare known per se for the transmission of packet data PD between themobile station MS and a packet data network PDN.

The method described below is based on the idea of selecting a robuststatistical approach for the received data. A statistical approach istermed robust here if it is insensitive to contaminations of the assumedGaussian distribution of the interference. That is to say “smallchanges” in the received data should also cause only “small changes” inthe resulting estimates. “Small changes” may be here both “large changesin a small fraction of the data” and “small changes in all data”. Thisproposed approach is intended to combine the advantage of maximum ratiocombination (MRC: Maximum Ratio Combining), namely great effectivenessin the non-contaminated case, and the advantages of robust estimationsor estimation operators in the contaminated case. For instance,diversity combining with equal level of gain (EGC: Equal Gain Combining)has a more stable behavior in a large class of noise distributions thana maximum ratio combination (MRC), but lower effectiveness in the(non-contaminated) case of Gaussian noise. It also requires noestimation of signal-to-noise ratios from the data.

With the maximum ratio combination MRC, the resulting data z are formedas the sum of the products of the received data x and y with theirsignal-to-noise ratios SNRx and SNRy respectively:z=SNRx*x+SNRy*y.

-   -   x here denotes the “older” received data from an earlier        transmission which the receiver has stored, and y denotes the        “more recent” received data of the last transmission received.

During the demodulation of received data, the data (PDU) modulated ontoa carrier signal prior to transmission are mapped onto in-phase andquadrature-phase data portions of the complex number level, so that thereceived data strings x and y which can be further processed as well asthe reconstructed data z are complex values.

In the case of Gaussian noise, following the diversity combination themaximum ratio combination MRC produces the maximum signal-to-noise ratioSNRz and the concomitant lowest block error probability. That is to say,the data z combined in this way can be decoded with the maximumprobability of success. However, the long error propagation memory ofthe maximum ratio combination MRC has a negative effect.

When a diversity combination with equal level of gain (EGC) is used, thearithmetic mean of the received data x and y is formed to reconstructthe data z:z=(x+y)/2.

In the case of Gaussian noise and different SNRx, SNRy, this produces alower signal-to-noise ratio SNRz, but is more stable than the maximumratio combination MRC with respect to fading or fluctuation effects andnon-Gaussian noise signals. With respect to the error propagation,however, there is also a long error propagation memory when diversitycombining with equal level of gain (EGC) is used.

In a first step, the two methods, maximum ratio combination MRC andestimation operators EGC, are combined into one method in orderultimately to obtain a robust maximum ratio combination. A parameter γ(gamma) is introduced for this purpose, such that the following applies:z=(1−γ)x+γy, where 0≦γ≦1.

The parameter γ may be suitably selected by the receiver here. In thecase where y=1/2, the corresponding calculation of the resulting data zleads to the same result as the use of diversity combining with equallevel of gain (EGC) known per se, and in the case whereγ=SNRy/(SNRx+SNRy) it leads to the same result as the use of the maximumratio combination (MRC) known per se. Depending on the selection of thefactor γ, alternately stability is improved or decreased as againsteffectiveness, so that the factor γ can also be referred to as stabilityparameter γ in the text below in order to distinguish it better.

To achieve an optimal result, the calculations are performed a number oftimes with a different stability parameter γ in each case. Given thecomputing power of even mobile subscriber stations MS today, this doesnot present any problems. The calculationz=(1−γ)x+γy

is preferably performed here with the following stability parametervalues, where

-   -   γ=0 corresponds to ignoring the more recent or last received        data,    -   γ=1/2 corresponds to performance of a pure EGC,    -   γ=SNRy/(SNRx+SNRy) corresponds to the performance of a pure MRC,        and    -   γ=1 corresponds to ignoring the earlier, i.e. first received        data.

The stability parameter γ that produced the highest signal-to-noiseratio SNRz, or the successful decoding respectively, is then selected.

To improve the robustness of the method, non-linear cleaning functions ψare introduced. The functions serve to remove outliers in the data, thatis to say in particular individual data values that deviate greatly fromthe values of the neighboring data or from a mean level, or to limittheir influence on the resulting data. Outliers in the data are “pulledin” in the direction of origin by the cleaning function ψ. The formulaz=(1−γ)ψ(x/s _(x))+γψ(y/S _(y))

can be used as an example for calculating the reconstructed data z.Examples of such functions are Huber, Tukey and Hampel functions, thatis to say, for example, a function which, when the cleaning functionψ_(H)(X) is plotted on the ordinate against the value x on the abscissa,proceeds coming from negative abscissa values on the ordinate value −1,then rises from the abscissa value −1 through the common zero pointlinearly to the value pair 1/1 and proceeds further on the ordinatevalue 1. In addition, the stability parameter γ where 0≦γ≦1 which isdependent on the data defines the compromise between stability andefficiency. Moreover, robust scaling parameters s_(x) and S_(y) can beestimated using standard methods, for example by a method fordetermining the median of absolute deviation (MAD).

For further enhancement, evaluation factors ε can be introduced in orderto rank more recent, new or recently repeated transmitted data y higherthan older, buffered data x. In particular, the effect of an errorpropagation can be countered in this way. For this purpose, the olderdata y are devalued with the aid of the evaluation factor ε as a smallconstant ε, where in particular 1>>ε>0, in comparison with the morerecent data x, which can be calculated, for example, using the extendedformulaz=(1−γ−ε)ψ(x/s _(x))+(γ+ε)ψ(y/S _(y)).

The method can of course be applied irrespective of the transmissiondirection, that is to say both when one of the mobile subscriberstations MS is the transmitting station and the base station BS is thereceiving station, and vice versa.

The receiving station (MS; BS) is here preferably a receiving device(MS; BS) of a communications system (UMTS, GPRS) for carrying out themethod described above. The receiving device (MS; BS) has a storage unit(S) for buffering data packets (PD) received a number of times from thetransmitter (BS; MS), and a calculation unit (C) for performing thecalculations in accordance with the formulae described above or otherformulae having a comparable effect.

Particularly preferred application areas are currently thecommunications systems having “hybrid” ARQ algorithms with thedesignations UMTS UTRA in TDD and FDD mode or TD-SCDMA mode for Chinarespectively, Fixed Wireless Access or HIPERLAN/2. The ARQ Type IIalgorithms increase the redundancy between individual transmissions andused diversity combination. In particular, the method can however alsobe realized in other communications systems that use a diversitycombination for antenna diversity, “soft handover/macro diversity” orthe like. The method is not limited here to the described transmissionvia the air interface V, but in principle can also be applied to otherinterfaces, for example to line-based interfaces.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1. A method for the transmission of packet data in a communicationssystem between a transmitter and a receiver, comprising: repeatedlytransmitting data packets from the transmitter; repeatedly receivingdata packets at the receiver as data received earlier x and datareceived later y, and forming reconstructed data packets z at thereceiver using a diversity combination technique to combine the datareceived earlier x and the data received later y, weighting the datareceived earlier x and the data received later y in the diversitycombination technique, in a range between a maximum ratio combinationand an equal level of gain diversity combination wherein to reconstructdata packets z, a stability parameter γ is used to weight the datareceived earlier x and the data received later y, in accordance with thefollowing:z=(1−γ)xγy, where 0≦γ≦1.
 2. The method as claimed in claim 1, whereinthe stability parameter γ depends on a statistical distribution of thedata received earlier x and the data received later y.
 3. The method asclaimed in claim 2, wherein the reconstructed data packets z areestimated a plurality of times with a plurality of respective differentstability parameters γ in each case.
 4. The method as claimed in claim1, wherein the reconstructed data packets z are estimated a plurality oftimes with a plurality of respective different stability parameters γ ineach case.
 5. The method as claimed in claim 4, wherein a stabilityparameter γ associated with a maximum signal-to-noise ratio SNRz isselected to form the reconstructed data packets z.
 6. The method asclaimed in claim 1, wherein the reconstructed data packets z areestimated a plurality of times with a plurality of respective differentstability parameters γ selected from the group consisting of γ=0, γ=1/2,γ=SNRy/(SNRx+SNRy) with SNRi being a signal-to-noise ratio of the datareceived earlier x and the data receiver later y, and γ=1.
 7. The methodas claimed in claim 6, wherein the reconstructed data packets areestimated with each of γ=0, γ=1/2, γ=SNRy/(SNRx+SNRy) with SNRi being asignal-to-noise ratio of the data received earlier x and the datareceiver later y, and γ=1.
 8. The method as claimed in claim 7, whereina stability parameter γ associated with a maximum signal-to-noise ratioSNRz is selected to form the reconstructed data packets z.
 9. The methodas claimed in claim 8, wherein in the diversity combination, robustscale estimation operators s_(x) and S_(y) are estimated and used toscale the data received later y.
 10. The method as claimed in claim 9,wherein a cleaning operator ψ, which is a non-linear function, is usedto limit deviating data values when reconstructing the data packets z,in accordance with the following:z=(1−γ)ψ(x/s _(x))+γψ(y/S _(y)).
 11. The method as claimed in claim 10,wherein an evaluation factor ε>0 is used to weight the data receivedlater y greater than the data received earlier x when forming thereconstructed data packets z, in accordance with the following:z=(1−γ−ε)ψ(x/s _(x))+(γ+ε)ψ(y/S _(y)).
 12. The method as claimed inclaim 6, wherein a stability parameter γ associated with a maximumsignal-to-noise ratio SNRz is selected to form the reconstructed datapackets z.
 13. The method as claimed in claim 1, wherein in thediversity combination, robust scale estimation operators s_(x) and S_(y)are estimated and used to scale the data received later y.
 14. Themethod as claimed in claim 13, wherein a cleaning operator ψ, which is anon-linear function, is used to limit deviating data values whenreconstructing the data packets z, in accordance with the following:z=(1−γ)γ(x/s _(x))+γψ(y/S _(y)).
 15. The method as claimed in claim 14,wherein an evaluation factor ε>0 is used to weight the data receivedlater y greater than the data received earlier x when forming thereconstructed data packets z, in accordance with the following:z=(1−γ−ε)ψ(x/s _(x))+(γ+ε)ψ(y/S _(y)).
 16. A receiving device in acommunications system, comprising: a storage unit to buffer receiveddata packets, each data packet being received a plurality of times froma transmitter; and a calculation unit to reconstruct the data of thetransmitted data packet by combining at least two copies of the datapacket using a diversity combination, the copies of the data packetbeing received at different times, the calculation unit producingreconstructed data z, wherein in the diversity combination, thecalculation unit weights data packets received earlier x and datapackets received later y in a range between a maximum ratio combinationand an equal level of gain diversity combination, and to reconstructdata packets z, a stability parameter γ is used to weight the datareceived earlier x and the data received later y, in accordance with thefollowing:z=(1−γ)x+γy, where 0≦γ≦1.
 17. A radio communications system, comprisingat least one receiving device, the receiving device comprising: astorage unit to buffer received data packets, each data packet beingreceived a plurality of times from a transmitter; and a calculation unitto reconstruct the data of the transmitted data packet by combining atleast two copies of the data packet using a diversity combination, thecopies of the data packet being received at different times, thecalculation unit producing reconstructed data z, wherein in thediversity combination, the calculation unit weights data packetsreceived earlier x and data packets received later y in a range betweena maximum ratio combination and an equal level of gain diversitycombination, and to reconstruct data packets z, a stability parameter γis used to weight the data received earlier x and the data receivedlater y, in accordance with the following:z=(1−γ)x+γy, where 0≦γ≦1.